The present invention relates to a device for detecting the presence or absence of a flame in an open type combustor and the oxygen density of the air in the room where the combustor is operated by detecting the ion current of the flame in the combustor.
The oxygen density of the air in the room where a heating apparatus or a water heater with an open type combustor using indoor air for combustion is installed is an essential factor for safety and health. Therefore, it is necessary to detect the oxygen density of the air in such a room and to stop the combustor or to ventilate the room is necessary.
As shown in FIG. 1, the ion current value of a flame in an open type combustor decreases with the oxygen density of air in the room. Therefore, the oxygen density of air in the room can be detected by detecting the flame ion current. The flame ion current value is detected to determine whether flame is present therein or not to provide a detection signal which is used to control the operation of the combustor. Therefore, if the flame presence or absence detecting device provided in the combustor is used as a density detecting device to detect the amount of oxygen in the air in the room, then it is not necessary to provide such a separate oxygen density detecting device. That is, flame detection and oxygen density detection can be carried out with a single device resulting in a reduction in cost.
However, as shown in FIG. 2, the flame ion current value is greatly pulsive for period of time after combustion starts. Therefore, in order to avoid erroneous detection of the presence or absence of flames, it is necessary to increase the output level of an ion current detecting sensor in a flame presence or absence detecting device for the open type combustor. If the output level is increased, the ion current level, which is relatively stable after a certain time, is also increased. Thus, it has been impossible to use a flame presence or absence detecting device as a room air oxygen density detecting device. The reason for this will be described in detail with reference to FIG. 3.
If the sensitivity of the ion current detecting sensor is not increased, the characteristic of flame ion current with time is as indicated by the dotted line in FIG. 3. Therefore, if the value C is selected as a reference value for determining the presence or absence of flame, it may be determined that flame is absent although actually flame is present. In order to eliminate this drawback, the sensitivity of the ion current detecting sensor can be increased to provide a characteristic of ion current level with time as indicated by the solid line in FIG. 3. In this case, the presence or absence of flame can be positively determined with the reference value C mentioned above.
However, if the sensitivity of the ion current detecting sensor is increased, then the ion current level after the start of combustion is also increased, as described above. Thus, at the time instant t.sub.1 when the oxygen density of the room air should be detected and when the oxygen density is sufficiently low as to present a health hazard, the ion current level does not decrease to the value C. The ion current level does not decrease to the value C until some time after the time instant t.sub.1. For this reason, it has been impossible to use the flame presence or absence detecting device as a room oxygen density detecting device. Thus, different devices have heretofore been required for detecting the presence or absence of flame and the oxygen density of room air resulting in a high total cost.